While two-dimensional area spatial light modulators such as transmissive and reflective LCDs offer significant promise for use as image forming devices in high-resolution printers, there are some inherent drawbacks to using these devices. Among these drawbacks is the necessity for employing some form of defect correction due to imperfect manufacturing processes. High-frequency defects occur, for example, as a result of surface brushing needed for providing an alignment bias to the liquid crystal structures. Low-frequency defects occur, for example, due to other irregularities across the surface of the area spatial light modulator.
One method of compensation for defects of this type uses a defect map along with an accompanying gain table. For example, commonly assigned U.S. patent application Ser. No. 10/360,030, filed Feb. 7, 2003 entitled A METHOD FOR DETERMINING AN OPTIMUM GAIN RESPONSE IN A SPATIAL FREQUENCY RESPONSE CORRECTION FOR A PROJECTION SYSTEM, by Bernardi et al. discloses a method for obtaining a defect map and accompanying gain table that can be applied to a printing system. The defect map/gain table combination developed using this method can be used to effectively compensate for various types of both low- and high-frequency defects peculiar to an individual LCD or other area spatial light modulator, over the range of achievable light intensity levels.
One problem with existing defect correction techniques relates to a dependence on proper device calibration. Particularly in printing applications using an area spatial light modulator, frequent recalibration of the spatial light modulator is necessary, for example to accommodate changes in device performance or to compensate for batch-to-batch differences in the response of photosensitive media. However, changes in calibration can be accompanied by unwanted changes in defect compensation. Thus, while defect correction cannot be made completely independent of device calibration, it would be advantageous to make defect correction as independent of calibration as possible, so that calibration changes have negligible effect on device defect correction.
Some of the problems that complicate obtaining effective defect correction from one calibration to the next are due to inherent limitations of printing apparatus hardware. For example, the spatial light modulator, although itself capable of imaging at resolutions of 10-bits or higher, may be limited by its support components to image data having a bit depth, also termed a “bit space”, of no more than 8-bits. Thus, imaging data and look-up tables (LUTs) may be constrained to 8-bit values. However, calibration algorithms can be more effective using a higher bit space, such as using 10-bit or 12-bit values. Because of inherent device limitations, values computed in a higher bit space often need to be subsequently translated to a lower bit space for support components of the spatial light modulator. As a result of this need to scale between higher and lower bit spaces, some image contouring and related effects can degrade imaging performance following calibration. Added defect correction can further degrade uniformity over some areas of the spatial light modulator, causing visible image aberrations due to over-correction of component defects. Over-correction of this type could cause streaking in a printed image, for example.
Thus, it can be seen that there is a need for an image processing path that minimizes potential negative effects of calibration on both low- and high-frequency defect correction and provides a high degree of imaging uniformity, making defect correction substantially independent of calibration.